High efficiency phononic crystal reflective gratings for surface acoustic waves

Phononic crystals (PCs) consisting of media arranged periodically perform acoustic band gaps and can stop surface acoustic wave (SAW) propagation efficiently with tens of lattice periods. Thus PC was designed as space-sparing wave reflectors for SAW and verified experimentally. In this paper, further analysis of SAW encountering PC is processed and a modified PC structure is proposed to improve the performance of reflective gratings. A silicon-based square lattice PC formed by cylindrical holes is adopted. With a 10 mu m lattice constant and a filling fraction of 0.283, the PC has a band gap of 194-223 MHz for SAW along the GX direction. Finite element method is used to analyze 210 MHz incident SAWs on different PC gratings. PCs composed of cylinders with gradually increasing radii, called tapered PCs, and with finite depth cylindrical holes are considered as reflective gratings and the reflection is evaluated by comparing the amplitudes of reflected SAWs. The calculation shows that a three-step tapered PC with a finite depth has a higher efficiency because the amplitude of SAW reflected from the tapered PC is 0.9 times of incident one and the multiplication number is 0.84 for a normal PC grating. Further a SAW resonator is constructed by the improved tapered PC gratings, and the maximum amplitude inside the cavity is two times larger than the peak inside the cavity defined by the normal PC gratings. Thus the modified PC grating reflects SAW more efficiently and helps to improve the performance of SAW devices.